well after lvl15 there sure is a wall in the exp you gain in ff14 imo. i can feel da grind now. Still enjoying my time with it, just got to lvl17, we'll be together soon my sweet chocobo (assuming there aren't any strings attached like, gil, cause i got like 4k tops)
Make sure you're joining FATEs regularly - they provide huge boosts in EXP for relatively little work. Stick to your story quests too, since the EXP rewards are double what they originally were. Once you're able to access the duty roulettes, you can almost jump an entire level through daily dungeon runs. I'm not sure how much work it involves, but the
Hall of the Novice has a ring reward that gives you 30% EXP gains until you reach level 30, which is no small amount! If you feel like leveling up a new class, the Armory Bonus gives you double EXP for kills, levequests and FATEs, but not quest rewards. Armory Bonus is only available when you're playing a class that isn't your highest level, so if you have no interest in other classes, it won't help you at all.
To completely optimize, you could use the
friend referral system to get the Friendship Circlet, which boosts EXP by 20%, and it stacks with the novice ring. Unfortunately, you'll need to actually pay for at least 30 days of subscription time for that, so it might not be an option for you right now.